1. Field of the Invention
This invention relates to an apparatus for measuring the refractive power of the eye. More particularly, it relates to an apparatus for preventing any variation in dimensions of the image of an optical pupil, i.e. the stop of an optical system, or the image of the iris of the eye when an optical system for projecting a measuring beam onto the eye fundus and an optical system for receiving the beam reflected by the eye fundus are focused to the eye fundus.
2. Description of the Prior Art
Eye refractometers have long been used to examine the function of the eye or to obtain data for making spectacles. According to the conventional eye refractometers, a test chart has been projected upon the eye fundus and focus adjustment has been effected while the image of this chart is being observed through the viewfinder so that the eye refraction is measured from the amount of adjustment. In contrast, apparatuses for automatically measuring the eye refraction have recently been proposed in U.S. Pat. Nos. 3,536,383; 3,819,256; 3,883,233 and 3,888,569 and these apparatus are characterized by a beam projecting portion for projecting a beam forming a chart image and a detecting portion for detecting the chart image reflected by the eye fundus.
The assignee of the present invention also has proposed, in U.S. Applications Ser. No. 944,304 and now U.S. Pat. No. 4,293,198 and No. 75,115 (German Application Pat. No. 2937891.4), an apparatus for measuring the refraction error including information on astigmatism. The examples shown in FIGS. 1 and 6 of the accompanying drawings are modifications of the embodiments described in the specifications of said applications.
Here, the constructions of FIGS. 1 and 6 will first be explained, and then the problems peculiar thereto will be pointed out. FIG. 1 refers to a case where the light-receiving surface is moved. In FIG. 1, reference characters 1a, 1b and 1c designate light sources, and reference character 2 denotes a mask having three slits 2a, 2b and 2c equidistant from the center thereof and perpendicular to meridians which form 120.degree. with one another, as shown in FIG. 2. The light sources are provided behind the respective slits. L.sub.1 designates a fixed lens, 3 an apertured mirror having apertures 3a, 3b and 3c equidistant from the center thereof and spaced apart from one another by 120.degree., as shown in FIG. 3, L.sub.2 an objective lens, E the eye to be examined, Ef the eye fundus, C the cornea of the eye, 4 a reflecting member, L.sub.1 ' a fixed lens similar to L.sub.1, 5 an aperture stop having an opening 5a as shown in FIG. 4, and 6 a light-receiving mask having slits 6a, 6b and 6c as shown in FIG. 5. The slits 2a and 6a, 2b and 6b, and 2c and 6c are in optically conjugate relationship with respect to the mirror 3. Designated by 7a, 7b and 7c are light receptors which can substantially cover the slit portions of the slits 6a, 6b and 6c, respectively. The light from the light source 1a passes through the slit 2a, the fixed lens L.sub.1, the aperture 3a of the apertured mirror 3, the objective lens L.sub.2 and the cornea C to the eye fundus Ef and forms the image of the slit 2a on the eye fundus Ef. The light reflected by the eye fundus Ef passes to the slit 6a on the light-receiving mask 6 via the cornea C, the objective lens L.sub.2, the apertured mirror 3, the reflecting member 4, the fixed lens L.sub.1 ' and the aperture stop 5, and is received by the light receptor 7a. This also holds true with the lights from the light sources 1b and 1c.
When the mask 2 and the light sources 1a, 1b, 1c and the mask 6 and light receptors 7a, 7b, 7c are simultaneously moved in one direction in synchronism with one another, the images of the slits 2a, 2b, 2c formed on the mask 6 which have been blurred at first and positionally deviated in the direction of the diametral line come to exactly overlap with the slits 6a, 6b, 6c and become clear and exhibit the extremal value of the quantity of light. Although not depicted in FIG. 1, it is to be understood that the amount of movement of the slit 2 is always being detected by position detecting means such as a linear encoder or the like. The position thereof on the optic axis corresponds to the refractive power.
Accordingly, if the apparatus is designed such that the output of the position detecting means is read when the light receptors 1a, 1b and 1c have detected the extremal value, it is possible to know the refractive power for each diametral line.
The refractive power of the eye to be examined is found by the use of the output received by the above-described method, and with the refractive powers actually measured with respect to three diametral line directions being defined as P.sub.1, P.sub.2 and P.sub.3, various amounts can be found by the use of the following equation. That is, when use is made of the spherical surface degree number A, the cylinder degree number B and the cylinder axis .phi., the equation P=A+B sin (2.theta.+.phi.) is established. .theta. is the angle of predetermined three diametral lines. Finally, A, B and .phi. can be simply evaluated by a calculation from the refractive powers in the three diametral line directions.
In the construction of FIG. 6, lenses movable in the direction of the optic axis are disposed instead of the fixed lenses L.sub.1 and L.sub.1 ' of FIG. 1, and the light sources 1a, 1b, 1c, the masks 2 and 6 and the light receptors 7a, 7b, 7c are fixed. The lenses L.sub.3 and L.sub.3 ' are once simultaneously moved in one direction during one measurement and, during this one scanning, the light receptors 7a, 7b, 7c pass through the extremal value and from the positions of the lenses at the time point of the extremal value, there are obtained the refractive powers for the three diametral line directions.
In the optical system as shown in FIG. 1, the opening 5a of the aperture stop 5 can always be maintained at a constant magnification in this optical system with respect to the eye E to be examined and therefore, it is possible to eliminate the fluctuation of brightness for diopter variation, but it is necessary to move the light source portion including the mask 2 and the light-receiving portion including the light-receiving mask 6 and the movement of the light source portion and the light-receiving portion is not linearly varied for the diopter variation of the eye to be examined and thus, the movement and correction thereof are cumbersome. On the other hand, in the optical system as shown in FIG. 6, fluctuation of brightness for diopter variation occurs and moreover, the movement of the lenses is not linear.